A Complete Analysis of the Key Technical Points of Seawater Pearl Farming

I. Artificial Seed Rearing
1. Selection and Conditioning of Parent Shells (Pinctada martensii)
(1) Select adult shells 2.5 to 4 years old, robust and thick-bodied, well-formed, vigorously growing, with intact shells, silver-white nacre, free of pests and diseases, and with plump gonads.
(2) In autumn, select the parent shells needed for the following year, and keep males and females separate for sparse stocking and deep hanging; in early spring, carry out shallow hanging for fattening. Indoor artificial warming for fattening is recommended, so as to achieve early seed rearing and early transfer to the sea.
(3) Male and female parent shells should be selected from different regions and different sea areas as distant-lineage shells, to avoid germplasm degeneration caused by inbreeding. In a planned way, starting from small shells, select those that grow fast, are large in size, thick-bodied and disease-free to raise as parent shells.
2. Induced Spawning and Artificial Insemination.
(1) Sperm and eggs obtained by artificial induction have a high degree of maturity, and the offspring raised are robust; moreover, the parent shells do not need to be killed. So the artificial induction method should be used as far as possible to release sperm and spawn eggs. Artificial induction methods include temperature stimulation, exposure to air and drying, a combination of light and temperature (such as sun exposure), electric current stimulation, changing seawater specific gravity, changing seawater pH, ammonia stimulation, ammonia injection, germ cell stimulation, etc. At present, the method commonly used is shade-drying with temperature change combined with ammonia seawater stimulation. The procedure is: shade-dry the parent shells for 2-3 hours, add ammonia to seawater to make 0.05-0.07‰ ammonia seawater, adjust the temperature with heated seawater, soak the parent shells at 29-30°C for 20-30 minutes, then transfer them into seawater at normal temperature.
(2) If the artificial induction method cannot achieve the goal, then use the dissection method for artificial insemination. Take the selected parent shells, wash them clean, cut them open, remove the right shell, cut off the byssus, mantle and gills to expose the gonads, wipe away body fluids and seawater with sterilized absorbent cotton, then use a slender pipette to draw the sperm and eggs, placing the sperm and eggs separately into beakers containing filtered seawater. Before insemination, first fill the insemination tank or aquarium with filtered seawater and prepare ammonia seawater at a concentration of 0.01-0.07‰, then pour the eggs filtered through silk gauze into the fertilization tank; after 5-6 minutes, pour in the sperm filtered through silk gauze, stir thoroughly to allow fertilization. The ratio of female to male shells is 3:1. The suitable water temperature for fertilization is 26-30°C, and the relative density is 1.0175-1.0240. Direct sunlight should be avoided during insemination and embryonic development. After fertilization, change the water 2-3 times; at the first water change, try to remove the ammonia seawater and excess sperm as completely as possible.
(3) The temporary rearing density of larvae depends on air temperature and water temperature, and should be controlled within 50-60 individuals/ml.
3. Daily Management
(1) Rearing density: before transferring into the pond, put about 1/3 filtered seawater into the seedling rearing pond in advance, then move the larvae in. A density of 2-4 D-shaped larvae/ml is appropriate.
(2) Adding and changing water: add an appropriate amount of fresh filtered seawater every day for 3 days, and fill it up on the 3rd day. Start changing water on the 4th day; the amount of water changed is 1/3 during the D-shaped stage, gradually increasing to 1/2-2/3 as the individuals grow. Before adding water each day, measure the seawater relative density and temperature; the relative density must not be lower than 1.0180, and the temperature difference of the added water must not exceed 2°C.
(3) Feeding: feed once each day in the morning after changing water and once at 3-4 pm. During the D-shaped larval stage, feeding should be small in amount but fine in quality. The feeding amounts are as follows:
Daily feeding amounts for Pinctada martensii larvae at different developmental stages:
D-shaped stage: Platymonas subcordiformis 300-500 cells/ml, Isochrysis zhanjiangensis 200-500 cells/ml, dry yeast 0.04-0.10 ppm; early umbo stage: Platymonas subcordiformis 500-1000 cells/ml, Isochrysis zhanjiangensis 600-1000 cells/ml, dry yeast 0.10-0.15 ppm; middle umbo stage: Platymonas subcordiformis 1000-1500 cells/ml, Isochrysis zhanjiangensis 600-1000 cells/ml, dry yeast 0.15-0.20 ppm; late umbo stage: Platymonas subcordiformis 1500-2000 cells/ml, Isochrysis zhanjiangensis 600-1000 cells/ml, dry yeast 0.20-0.15 ppm; attachment and seedling stage: Platymonas subcordiformis 2000-3000 cells/ml, Isochrysis zhanjiangensis 1000-3000 cells/ml, no dry yeast; seedling to harvest stage: Platymonas subcordiformis 3000-4000 cells/ml. No others are fed.
(4) Aeration: use an air compressor for continuous aeration.
(5) Other management work: every day check and observe the larvae's activity, feeding, and growth and development, keep good records, maintain good water quality, and prevent and remove diseases and pests.
4. Selection and Placement of Collectors: for the collectors, it is best to use darker-colored plastic plates, such as black or red ones. The collectors need to be soaked in filtered seawater for one week, then disinfected with 5 parts per million bleaching powder or potassium permanganate before use. When about 20% of the larvae show eye spots, begin placing the collectors. Place 100-150 pieces per cubic meter of water, completing the placement in 2-3 batches within 3-4 days.
5. Harvesting Seedlings: 20-30 days after the seedlings attach, when the individuals reach 2 mm-3 mm and meet commercial specifications, they can be harvested. The seedlings must be sorted into cages and transferred to the sea for farming on the same day they are harvested.
　　
II. Management and Care of Pearl Oysters
The management and care of pearl oysters is an important part of pearl farming production; it affects the number and quality of nucleus-inserted oysters and the yield and quality of pearls.
1. Site Selection: choose the middle part of a bay or a bay mouth with small waves that has good wind shelter, smooth tidal currents, and abundant plankton; for the farming water layer used, the water temperature in winter should not be lower than 12°C, and in summer not higher than 31°C; it is best if the bay has an appropriate amount of freshwater inflow, but in the rainy season the seawater specific gravity should not be lower than 1.0150; for bamboo raft farming, the water depth at lowest tide should be 5 m; for floating longline raft farming, the water depth at lowest tide should be 7 m; for fixed hanging-culture farming, the water depth at lowest tide should be 2 m; the bottom should be sand or sandy mud; there should be few harmful organisms; and there should be no adverse effects from factory or urban sewage.
2. Management and Care of Various Types of Pearl Oysters
(1) Seedlings (shell length under 3 cm): this is an important stage in mother-of-pearl oyster farming. In management, one should check frequently, remove pests and enemies frequently, clean the seedlings and cages frequently, and carry out sparse stocking and cage changing at the proper time. When the seedlings vary greatly in size, they should be sorted by size; floating raft farming is best.
(2) Small and medium oysters (shell length 3.1-6.9 cm): do a good job of sparse stocking and cage changing, remove attachments to promote growth, and strive to raise them into large oysters in a short period.
(3) Large oysters (shell length over 7 cm): this is another important stage in pearl oyster farming; it determines the number and quality of large oysters supplied for nucleus insertion. One must choose a site with smooth water flow, abundant feed, and good conditions for wind protection, cold protection, heat protection, freshwater protection, and pest protection, and use a suitable water layer for management and care.
(4) Resting oysters: the purpose of resting is to let the operated oysters recover their health and prevent their death; through resting, the pearl nucleus does not move, or moves less, before the transplanted piece forms the pearl sac, so as to prevent nucleus loss and the occurrence of plain pearls and deformed pearls. The resting period is 20-25 days. The resting site requires calm winds and waves, smooth water flow, small changes in water temperature and seawater density, water depth (at lowest tide) of more than 3 m, a relatively hard bottom, and relatively low water temperature; floating raft farming is best. The resting cage uses a double-ring net cage, with net cloth fitted to the bottom of the cage; it is hung for rearing in a water layer below 2.5 m, and the stocking density is slightly increased compared with oysters of the same type.
(5) Pearl-culturing oysters: this is the final stage determining the success or failure of pearl farming. One must choose the best sea area and site, strengthen the health management of the pearl-culturing oysters, do a good job in pearl quality management, grasp the most suitable farming water layer, and frequently clean off attachments and sludge. During operations, movements should be light and quick. The time of exposure to air should be short, and they must not be taken onto land for shell cleaning.
　　
III. Preoperative Treatment
The purpose of preoperative treatment is to physiologically suppress the operated oysters and at the same time avoid poor nucleus insertion results caused by full gonads. After preoperative treatment, the operated oysters have a lower mortality rate after nucleus insertion and the pearl quality is better.
1. Method of Inhibiting Gonadal Development: this method should be carried out after a period of recovery following the reproductive season in the winter of the previous year before the operation. The commonly used net cage treatment method: the oysters are treated with 30-mesh polyethylene net cloth bags, with a mesh of 2 mm, and the net bag is 100 cm long and 50 cm wide. Use the net cloth bag to cover the oyster cages, with 2 cages of oysters per bag and 40 oysters per cage; tie the mouth of the net cloth bag tightly with rubber thread rope, and hang it for rearing in a water layer 1-2 m deep under the floating raft.
2. Method of Promoting Gonadal Maturity and Advancing the Release of Sperm and Eggs: this method is the opposite of the inhibition method; it uses promotion of gonadal maturity and early release of sperm and eggs to obtain operated oysters. The commonly used layer-changing method: after the water temperature rises back to 17°C in spring, hang the sparsely stocked mother oysters in a water layer of 1.5 m where the water temperature is higher, which can promote gonad maturity; after the water temperature reaches 22-24°C, clean the oysters and put them into net cages with smaller mesh, hanging them in a water layer of 5 m to keep them in a resting state. On a clear day before the operation, change them to a 1 m water layer; before long they release sperm and eggs. The next day, lower them again to a depth of 5 m; after 5-7 days, when the climate and sea conditions are suitable, carry out a second shallow hanging for induced spawning. At this time about 60-80% can be used for the operation; those temporarily unusable can be stored in deep water below 5 m to prevent the gonads from developing again, and on the day before use, raise them to a 2 m water layer for induced spawning. Besides the layer-changing method, there are also the pond water induced spawning method, the ground sowing method, the sun exposure method, and the electric stimulation method.
3. Rest and Recuperation of Oysters: after treatment, the oysters' physiological state and gonads have basically met the requirements, but the oyster bodies are very weak, so they still need a rest and recuperation period. The rest and recuperation period is generally about one week. During rest and recuperation, different measures should be taken according to the specific condition of the oysters, so that the oyster bodies can recover appropriately.
　　
IV. Nucleus Insertion
Nucleus insertion is an important part of pearl farming production; it directly affects the yield and quality of pearls. Nucleus insertion should be carried out in March each year when the water temperature rises above 18°C. During the nucleus insertion process, if the water temperature exceeds 31°C or the seawater density is lower than 1.0150, the work should be suspended.
1. Oyster Arrangement and Mouth Pegging: 2-3 weeks before oyster arrangement, remove attachments; before arranging the oysters, first wash off floating mud. The oyster arrangement time is generally 2-4 hours; if the arrangement time is insufficient, one must not force the oysters to open for pegging. When using methods such as dense arrangement with flowing water, attention should be paid to shortening the time. Mouth pegging should be done so that the nucleus position can be seen clearly and operation is convenient, without damaging the adductor muscle.
2. Selection of Operated Oysters and Piece Donor Oysters: operated oysters are preferably 2-3 years old, with a shell length over 7 cm. Piece donor oysters are preferably 2-3 years old, with a shell length over 7 cm, the nacre on the inner surface of the shell iridescent or silver-white, the mantle moderate in thickness, with not much pigment or white spots at the edge, and the oyster body robust.
3. Preparation of the Piece: the procedure for preparing the piece is: take the piece - spread it flat on wet gauze - wipe the piece - cut into long strips - soak in a mixed solution of PvP (or other chemicals) and mercurochrome for 3 minutes + cut into squares, keep the pieces in filtered seawater - use.
4. Nucleus Insertion
(1) Incision position: the incision on the right side of the oyster body should be at the base of the foot (that is, the black-white boundary). The incision on the left side of the oyster body (when inserting the lower-foot nucleus position) should be 1 mm from the base of the foot; the incision is arc-shaped, slightly narrower than the diameter of the pearl nucleus; the incision should be thin. Take care not to cut the byssal gland; during the operation, attention should be paid to cleanliness and hygiene at the incision site.
(2) The width of the channel is slightly smaller than the diameter of the pearl nucleus, and the depth must not exceed the nucleus position.
(3) Nucleus positions: there are the left pocket, right pocket, and lower foot.
(4) When delivering the piece, the ten-piece needle should pierce the front 1/3 of the piece. The outer side (front side) of the piece must be pressed entirely tight against the pearl nucleus; the piece should be stuck on the side opposite the intestine or the retractor foot muscle behind the pearl nucleus.
(5) Nucleus delivery: after the pearl nucleus enters the incision, use a small nucleus delivery tool to send the pearl nucleus to the nucleus position.
(6) The amount of nucleus inserted depends on the size of the oyster and the specification of the pearl nucleus. Generally, for 8-10 mm pearl nuclei, insert 1 per oyster; for pearl nuclei below 8 mm, 2 may be inserted.
　　
V. Rest and Recuperation of Operated Oysters and Harvesting of Pearls
Pearl oysters suffer relatively great trauma during nucleus insertion; if handled improperly, it can cause death. Therefore, after the operation, a rest and recuperation period of more than ten days to one month is needed, so as to help the wounds of the operated oysters heal and restore normal physiological activity. Before harvesting pearls, first conduct sampling inspection of the pearl-culturing oysters. If the measured nacre thickness reaches the standard, the oysters can be opened to harvest the pearls.